Polar Bear Farm’s aptly named Tweet Push is another option for receiving Twitter push notifications on the iPhone. Like the aforelinked Boxcar, it isn’t a full Twitter client itself, but rather acts as a companion to existing iPhone Twitter clients (currently: Tweetie, Twitterrific, Twittelator, and TwitterFon; I’d love to see Birdfeed added to the list).

Tweet Push costs just $1, and includes a month of service. After that, each month costs another $1 via in-app purchasing. Tweet Push offers fine-grained settings — you can turn on notifications for direct messages, mentions, and even your main timeline (presumably for people who follow only a handful of other Twitter users, or, perhaps, for the insane). I’ve been trying it this morning and I like it.